The invention relates generally to thermal ink jet printing and, more particularly, to printheads with resistive heaters provided with improved drop ejection efficiency.
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. An ink nucleation process is initiated by individually addressing resistors with short (2-6 .mu. second) electrical pulses to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The environment of the heating element during the droplet ejection operation consists of high temperatures, thermal stress, a large electrical field, and a significant cavitational stress. Thus, the need for a cavitational stress protecting layer over the heating elements was recognized early, and one very good material for this purpose is tantalum (Ta), as is well known in the industry.
It has been demonstrated that nucleation efficiency is dependent upon the properties of the heater surface. (See article by Michael O'Horo et al. entitled "Effect of TIJ Heater Surface Topology on Vapor Bubble Nucleation", SPIE Journal, Vol. 2658, pgs. 58-64, Jan. 29, 1996). In this article, experimental observation showed that vapor bubble nucleation consisted of two types; homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation occurs in the ink spontaneously when the nucleation temperature is reached. Heterogeneous nucleation usually occurs at surface sites (cracks and crevices) of the resistive heater. The surface sites contain trapped gases or vapors which cause the initiation temperature for heterogeneous nucleation to be considerably lower than that of homogeneous nucleation. The stored energy and consequent efficiency of vapor bubble expansion is significantly reduced.
The preferred material for resistive heaters is polysilicon, or other sputtered resistor materials. Polysilicon is comprised of numerous grains whose size and roughness varies with high temperature cycling and doping levels. Polysilicon surface roughness for a high dose implant heater (heater 2 described in the O'Horo article) is 27.2 nm. The resistive heater is passivated with either a thermally grown oxide layer or pyrolytic CVD deposited silicon nitride, both of which are conformal; e.g. reproduce the polysilicon surface roughness on the surface of the passivation layer. A layer of tantalum is sputtered onto the passivation layer, which substantially replicates the underlying topography, as well as adding some topography due to the Ta grain structure. Therefore, the surface of the tantalum layer reproduces the surface side and hence, roughness of the underlying polysilicon and the nucleation efficiency of a heater structure of this type (polysilicon with passivation layer and tantalum) is not optimum.
From the above, it is evident that a smoother surface of either the polysilicon, or the passivation layer, or the tantalum would increase nucleation efficiency by reducing the number of vapor-trapping cracks or crevices. U.S. Pat. No. 5,469,200 discloses techniques used to polish the heater substrate to improve flatness and, in another example, to form a thermal oxide by oxidizing the substrate surface concurrently with a thermally softening step, resulting in a smoother surface on the oxide passivation layer. These techniques are not entirely satisfactory because of the excessively high temperatures and/or long heating cycles, resulting in incompatibilty with integrated microelectronics circuitry.